People who would be reluctant to provide metadata most of the time do so on Flickr because there’s a payoff for them. A) other people see their work–work is probably the wrong word because I don’t think most people see it as work in a serious artistic sense– but people see what they’re up to, see what they’re creating. And B) because they derive some pleasure from building value in the global collection.
I think this point is often missed by those who hold the “no cheap metadata” position. Turning “metadata” keywords into dereferencable URIs – as Flickr (among others) does – provides the means by which these “payoffs” that Stewart refers to, can be realized, cheaply. For me, it also helps drive home the point that one person’s metadata is another’s data, as this feedback loop is exactly the same one that exists when you publish any data behind a dereferencable URI. Taking it further, one might, for example, provide metadata for the Flickr metadata, which this is, if you squint a bit (not literally 8-). And to complete the loop and emphasize the point, one could easily republish many of these new images as new squared circles. Fractal!
As pointed out by Edd, Stewart also had this to say about Web services/REST;
I think we had one person inquire about using the SOAP version of the API. I don’t know if any apps were actually built. There is at least one application built on XML-RPC. But all the others–I don’t even know how many there are–are built on the REST API. It’s just so easy to develop that way; I think it’s foolish to do anything else.